The invention relates to a non-contact proximity switch with an oscillation circuit having a surge-like or step-like activation.
In a known proximity switch of this type (DE-OS No. 3318900) which reduces energy consumption, the pulse-like activation of the oscillating circuit is accomplished by periodic impulses, the spacing of which from each other in time is larger than the maximum release or decay period of the oscillations of the oscillating circuit. The average value of the rectified oscillations of the oscillating circuit or the amplitude of the releasing oscillations at a given point in time after the trigger impulse serves as the criterion for a switching operation. In this type of proximity switch, the precision and reproducibility of the switch position of the switch element that releases the switching operation is also not greater than in commonly used non-contact proximity switches which are based on the fact that the approach of a magnetic and/or electrically conductive particle or switch element effects by means of the magnetic field the oscillating conditions of the high frequency oscillator or affects a control value used in maintaining the high frequency oscillation in such a manner that a switch signal is produced. This is based on the fact that (a) a atime integration is necessary due to the small signal changes at the desired large spacing between the switch element and the oscillator, and (b) the attainable precision is limited by circuit component temperature dependence and insufficient miniaturization to a value which has often precluded the possibility of employing a proximity switch of this type.
In another known proximity switch (DE-OS No. 32 04 405), aside from an oscillator having a frequency which can be varied by an approaching switch element, a second oscillator with a nonvariable frequency is provided, the two frequencies are determined by means of two counters and compared by means of a comparator, which, in turn, releases a switch signal when a predetermined difference is found. The inductivity and frequency changes, which depend on the position of the switch element are so small that the frequency comparison does not lead to improved precision and reproducibility of the switch conditions.
With another known proximity switch (DE-OS No. 31 31 490) it is possible either to differentiate between a defined switch element, for example a trip dog in a trip rail and the approach of a metal particle such as a non-switch element metal chip, or between different control cams in a single groove field. In other respects, this proximity switch is not superior to the other known proximity switches.